In my website account on Google Webmasters tools, some search queries show average position 1.0. This make me understand that it should be displayed as the first result. When I search for this query I could not able to find my website's page listed as a result?! In some cases I navigate to the third or the fourth result page and I could not find it!

What are factors that make my website loss its average position for a search query? and when Google webmasters tools updates their values?